TECNOLOGÍAS COMO MEDIO PARA CONSTRUIR CIENCIA CON LA

Much research has been performed on the applications of impulse excitation in

regards to the various materials tested and the various material aspects that can be analyzed.

[36,40,43,44,45] The conclusions of earliest IE research led to the standardized testing

methods such as ASTM 1875 and 1876. These standards were specified for use with

isotropic elastic materials at ambient temperatures, however, recent studies have been

Computation of theoretical transverse vibration of bars with uniform cross-sections

performed by Timoshenko and Pickett led to the experimental determination of flexural

and torsional resonant frequencies of isotropic bars. [46,47] Timoshenko was one of the

pioneers that investigated the effect of shear deformation and rotational bending on the

dynamic motion of beams of uniform cross-section under various boundary conditions

(simply supported, free, clamped, etc.). [46] In the late 1950’s, Spinner performed rigorous

experimental testing of Timoshenko’s theories and found good agreement with the results.

[48] It is from this research that the ASTM standards for the use of resonant frequency

analysis and impulse excitation are based.

With the creation of the steam turbine and other high speed machines, it became

apparent that a method for determining fatigue of the rotating components was needed.

[49] Metals were the primary material used in the high speed turbine components and were

the first types of material that vibration analysis was performed on. In 1939, Rathbone, a

plant engineer, made the observation of the displacement of certain rotating shafts at low

frequencies and correlated these to the integrity of the shaft. [50] Empirical observations

such as this would eventually lead to the experimentation performed by Spinner to validate

previous vibration theories.

As technology advanced and more complex computational tools became available,

investigation of more than a materials resonant frequency was performed. Impulse

Excitation permits the non-destructive testing of material for frequency and internal friction

(damping properties) in harsh environments that would otherwise disrupt the use of other

NDT methods. In regards to metals, IET is a common method for testing. Heritage

at elevated temperatures. [51] It was concluded that under certain configurations, wave

guides can be used to consistently deliver the vibration signal of the sample to the acoustics

microphone. These findings correlated well with mechanical testing and piezoelectric

ultrasonic composite oscillator technique testing of the pure Aluminum. Radovic

performed similar IET on aluminum and steel samples of various geometries and found

good correlation between the resulting moduli and those found by resonant ultrasonic

spectroscopy and 4 point-bending tests. [52] Swarnakar and Jung performed IET on TiB2

and structural steel, respectively, and each found damping peaks that correlated to phase

changes in the metallic microstructure. [53,54] Another study involving the damping

behavior of material structural integrity of metals was performed by Goken. It was

determined that an increase to material damping resulted from crack growth during

prolonged heat treatment in magnesium alloy. [55]

In addition to metals, polymers are an excellent material to perform IET on due to

their viscoelastic nature. [45] A viscoelastic material exhibits the characteristics of both

viscous and elastic materials in which a viscoelastic material will dissipate stored energy

during unloading in the material’s elastic stress range. As a result, polymers have become

suitable interface and matrix material for high damping composites. Finegan noted various

enhancements to composite materials that would improve damping properties. It was found

that various layering configurations of polymer interfaces in a composite yielded

reasonably high damping. In addition, it was mentioned that co-curing of embedded layers

and hybridization of laminae under various fiber orientations enhanced the loss factors of

Much study has been performed in regards to nondestructive testing of ceramics

via resonant frequency analysis and impulse excitation. [36,57] Despite having low

damping characteristics, monolithic ceramics are excellent subjects for nondestructive

testing due to their primary failure mode of micro-cracking. Ceramics tend to have high

thermal resistance and are therefore used for various high temperature applications, such

as turbine and engine structural components. As a result, non-destructive testing can be

applied to ceramic materials exposed to high temperature testing via IET. Studies

performed by Roebben greatly involved exposing ceramics to elevated temperatures and

initializing IET on the samples to validate internal defects and phase alterations. The same

Resonant Frequency and Damping Analyzer (RFDA) used in these studies was used in

experimentation performed for this work. In one of the studies, Roebben investigates the

effect of elevated temperatures on the damping characteristics of oxide (Al2O3 and ZrO2)

and non-oxide (Si3N4) ceramics. The findings indicated that little frequency and damping

changes occurred below 1000o F, however, quickly progressed at higher temperatures due

to mobility of grain boundary defects and softening of secondary material phases in Al2O3.

[36] Damping peaks were found for ZrO2 that corresponded to the thermally activated

displacement of compensating oxygen vacancies that were present throughout the materials

thermal loading. In this same paper, damping peaks were observed during the loading and

unloading cycles of the Si3N4 due to the crystallization of intergranular phase of the non-

oxide ceramic. Another study by Roebben on IET of SiC and Si3N4 ceramic samples found

that variations of the annealed composition affected the damping amplitude of material

damping peaks. [58] Bemis performed IE of thermal shocked monolithic SiC rings cut from

temperature and impulse after each heating and quenching cycle for resonant natural

frequency and damping collection. [44] After first heat cycling to 250o _{C, only a damping}

change of 0.3 % occurred, however, a damping change of nearly 50.0% was found after

the ultimate cycle to 800oC. It was observed that visible cracking began after quenching

cycle from 400oC to room temperature. Although moduli was not determined from this

experimentation, it can be clearly seen that IET provides useful health monitoring for

thermally shocked materials prone to material cracking under rapid temperature changes.

Impulse Excitation has been of particular interest in regards to testing composite

materials. As mentioned in previous sections, damping characteristics are greatly

influenced by a materials micro-structure. [36,58] Considering that composites experience

a variety of micro-damage (micro-cracking, delamination, fiber break, etc.), this enables

IET to be an excellent method for non-destructive testing as seen from the recent research

performed. [41,42,43,59] Experimentation on glass and graphite fiber composite cantilever

(fixed-free) beams was performed by Crane, in which changes in loss factor was correlated

to changes in beam length. Crane determined that increases in beam length resulted in an

increase of loss factor and therefore the damping properties of each of the composites. [41]

A significant point behind this study was that it showed how impulse excitation testing

could be performed on a sample held in a cantilever mode of vibration and still produce

reliable results.

In a study conducted by the National Aeronautics and Space Administration

(NASA), IE on various graphite/ epoxy composites for Dynamic modulus determination

was compared with conventional mechanical testing and laminate theory. [40] This study

materials and emphasized how the method had potential for concurrent quality control. Atri

conducted IET of MMC composites for elastic modulus determination. These samples

consisted of discontinuous titanium-monobromide (TiB) whiskers of random orientation

imbedded in a Ti matrix with volume fractions ranging from 30 -83% TiB. Calculations

for moduli (Young's modulus, Shear modulus, and Poisson's Ratio) followed equations

implemented by those in ASTM standard 1876. A trend was found in which as TiB whisker

volume fraction increased, Young's and Shear moduli for the composite increases as well.

Properties calculated from IET correlated strongly with those of conventional mechanical

testing for all samples. Impulse Excitation can effectively be implemented on composite

material and can be confidently used in this paper.